The present invention relates in general to a system for transmitting/receiving an electrical signal, and more particularly to a system for transmitting/receiving a signal having a carrier frequency band for a radio base station.
In mobile communication of carrying out the communication utilizing an electromagnetic wave, in order that the limited frequencies may be repeatedly used at high efficiency and also the capacity of subscribers which are accommodated by a system may be increased, there is employed a method, called the cellular method, wherein the range of an electromagnetic wave which has been radiated from one base station is focused by sharpening the directivity of an antenna and the same frequency is spatially reutilized. In the current method, from the demand for miniaturization of the terminals, the information transmission amount which is required by the system, and the spatial attenuation characteristics of an electromagnetic wave, in the cellular mobile communication system, the range of the electromagnetic wave which has been radiated from a base station is in the range of about several km to about 20 km, the used frequency is in the range of several hundreds MHz to several GHz, and the radiation power of one terminal is in the range of several hundreds mW to about several W. Under such circumstances, in order that one base station may support the subscribers of several tens to several hundreds, the mean transmission electric power of several tens W to several hundreds W is required for the output of a transmission power amplifier of the base station. In addition, in order to realize the various communication services, currently, the mobile communication system for which the digital communication method is adopted is the main current. In this case, since the modulation method becomes the phase/amplitude modulation, the high linearity is required for the transmission power amplifier, and since the non-linearity of a semiconductor device used to realize that power amplifier needs to be compensated for, in general, the efficiency of the power amplifier of interest can not be enhanced, and the saturation output of several hundreds W to several kW is required for the power amplifier of interest. Then, it is desirable that the transmission power amplifier of the base station is installed in the vicinity of the associated antenna. If the electric power is supplied from the transmission power amplifier installed adjacent to an indoor base station at more than ten meters above the ground to the antenna which is installed at the height of 100 m, normally, the electric power loss of about 5 to about 6 dB occurs due to the loss of a high frequency cable such as a coaxial cable through which the antenna is connected to the transmission power amplifier. In order to compensate therefor, the output of the transmission power amplifier needs to be increased by about 5 times.
On the other hand, in order that the electromagnetic wave which has been radiated from a base station may be focused within a service area of one base station, the large gain is required for the antenna. In order that the transmission output may be focused within the service area having a diameter of about several m to about 10 km as in the current cellular system, the antenna gain of about 10 dBi to about 16 dBi is required. The gain of the antenna which can be realized by a single element is about 2 dBi, and hence in order to realize the larger gain of 10 dBi to 16 dBi, the technology, for an array antenna, of arranging a plurality of such single elements to increase the gain is required. In order to realize the gain of about 10 to about 16 dBi, the array in which eight to twelve single elements are arranged is required. In addition, in the normal operation of the system, by adjusting the area for which the base station provides the service, i.e., the area within which the electromagnetic wave is focused along with increase or decrease of the number of subscribers, the frequency utilizing efficiency of the overall system is enhanced. For this reason, the main beam of the antenna which is applied to the base station needs to be tilted downwardly and also the function of changing the tile angle is required.